This invention relates generally to a method for characterizing a sample composed of one or more thin films through the use of short pulses of electromagnetic radiation. A light pulse is absorbed at the surface of the sample. The absorption of the light pulse results in a time-dependent change xcex94R(t) in the optical reflectivity of the sample, and this change is measured by means of a time-delayed probe bean pulse. Analysis of the measured xcex94R(t) is used to deduce the crystallographic orientation of the grains in the sample.
Currently, in the semiconductor industry there is a great interesting the characterization of thin films. Integrated circuits are made up of a large number of thin films deposited onto a semiconductor substrate, such as silicon. The thin films include metals to make connections between the transistors making up the chip, and insulating films to provide insulation between the metal layers (see: S. A. Campbell, The Science and Engineering of Microelectronic Fabrication, Oxford University Press, (1996)). The metal films (interconnects) are typically arranged as a series of patterned layers. At the present time there may be 4 or 5 layers of interconnects. It is likely that as more complex integrated circuits are developed which will require a greater number of interconnections, the number of layers will increase. Metals of current interest include, for example, aluminum, cobalt, copper, titanium, and suicides. Insulating films include, for example, oxide glasses of various compositions and polymers.
In the production of integrated circuits it is essential that all aspects of the process be controlled as closely as possible. For metal films, it is desirable to measure properties such as the film thickness, the electrical resistivity, the grain size, the grain orientation, and the roughness of the surfaces of the film.
Currently available techniques for the determination of grain orientation include the following.
1) Electron Back-Scatter Diffraction. A sharply-focussed electron beam is directed onto the surface of the sample at an oblique angle. The back-scattered electrons diffracted from the atomic planes within the sample are detected. The intensity of these electrons form characteristic patterns, referred to as Kikuchi lines. From the angular positions of these lines, the crystallographic orientation of the atoms in the region of the sample 10 where the electron beam is incident can be determined. It is possible to scan the electron beam across the sample, and to determine how the crystallographic orientation varies from grain to grain. In this way the distribution of grain orientations can be obtained. This method has the disadvantage that a considerable amount of time is required to make-a measurement. In addition, the sample 10 has to be placed into a high-vacuum chamber for the measurement to be made.
2) Transmission electron microscopy. In this technique the diffraction of high energy electrons passing through the sample is measured. The grain orientation can be determined from the diffraction pattern. To make this type of measurement, it is essential to reduce the thickness of the sample so that high energy electrons can be transmitted. Thus, the method suffers from the disadvantage that the sample is destroyed. A second disadvantage is that the preparation of the sample takes a considerable amount of time. A third disadvantage is that the sample has to be placed into a high-vacuum chamber for the measurement to be made.
3) X-Ray Diffraction: In this technique X-rays are directed onto the surface of the film, and the diffracted X-rays are detected to determine the grain orientation. This method cannot be used for rapid measurements of grain orientation.
It is a first object of this invention to provide a method for the rapid determination of the grain orientation in a metal film.
It is a further object of this invention to determine the grain orientation without making contact to the film or causing the destruction of the film.
It is a further object of this invention to determine the orientation of the grains in a sample, to calculate the sound velocity for this orientation, and then to use this sound velocity together with a measurement of the time for a strain pulse to propagate through the sample in order to determine the film thickness.
In accordance with a first embodiment of the present invention, a non-destructive system for characterizing a thin film is provided, including a non-destructive system and method for measuring at least one transient response of a structure to a pump pulse of optical radiation, the measured transient response or responses including at least one of a measurement of a modulated change xcex94R in an intensity of a reflected portion of a probe pulse, a change xcex94P in a polarization of the reflected probe pulse, a change xcex94xcfx86 in an optical phase of the reflected probe pulse, and a change in an angle of reflection xcex94xcex2 of the probe pulse, each of which may be considered as a change in a characteristic of a reflected or transmitted portion of the probe pulse. The measured transient response or responses are then associated with at least one characteristic of interest of the structure.
The non-destructive system comprises a stage, an optical system for applying an optical pump pulse and an optical probe pulse to the film, a detector for detecting a change in optical response with respect to time, and a mechanism for changing strain in the thin film. The stage holds a substrate of a predetermined thickness and the film is located on a first side of the substrate. The optical system applies the pump pulse and the probe pulse to a free surface of the thin film such that the probe pulse is temporally delayed from the pump pulse. The detector detects the change in optical response with respect to time of the probe pulse reflected from the surface of the thin film. The mechanism for changing the strain in the thin film changes the strain from an initial strain value to a different strain value.
In accordance with a first aspect of the present invention, a method for the determination of grain orientation in a film sample is provided. The method includes the steps of measuring a first change in optical response of the film, and analyzing this change into components arising from 1) the change in the electron distribution that occurs shortly after the application of the pump pulse, 2) the propagation of strain pulses in the film, and 3) the change in temperature in the film. From these components of the change in optical response, the grain orientation in the sample is determined.
In accordance with a second aspect of the present invention, a method for the determination of the thickness of a film sample is provided. The grain orientation of the sample is first determined. The grain orientation, together with the velocity of sound and a propagation time of a strain pulse through the sample, are then used to determine the thickness of the film sample.